In order to find the natural substrate of the quinone dependent phosphatase from Clostridium sticklandii, protein components of the C. sticklandii glycine reductase system and E. coli glutamine synthetase were phosphorylated enzymically and tested as substrates. ThirtytwoP-labelled cell free extract from Clostridium stickllandii also was analyzed for the natural substrate. Immobilized selenocystamine was prepared as a selective affinity adsorbent of selenoprotein A and other selenium containing ligands.